1. Field of the Invention
The present invention relates to a method of reversing the tone of an image to be printed in a layer of radiation sensitive material formed on a substrate by controlling an illumination system adapted to produce multipole, small sigma and annular illumination arrangements.
2. Description of the Related Art
The term “patterning device” as here employed should be broadly interpreted as referring to device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device. An example of such a patterning device is a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase shift, and attenuated phase shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Another example of a patterning device is a programmable mirror array. One example of such an array is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be seen, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT publications WO 98/38597 and WO 98/33096. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
Another example of a patterning device is a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (IC's). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step and scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be seen, for example, from U.S. Pat. No. 6,046,792.
In a known manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement and/or inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemical, mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. It is important to ensure that the overlay (juxtaposition) of the various stacked layers is as accurate as possible. For this purpose, a small reference mark is provided at one or more positions on the wafer, thus defining the origin of a coordinate system on the wafer. Using optical and electronic devices in combination with the substrate holder positioning device (referred to hereinafter as “alignment system”), this mark can then be relocated each time a new layer has to be juxtaposed on an existing layer, and can be used as an alignment reference. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens.” However, this term should be broadly interpreted as encompassing various types of projection systems, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens.” Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. Nos. 5,969,441 and 6,262,796.
In order to keep pace with Moore's law and develop features having subwavelength resolution, it has become necessary to use a variety of resolution enhancement techniques (RET). The resolution limit of photolithographic techniques using a reduction exposure method is R=k1λ/(NA), where R is the resolution limit in nanometers (nm), k1 is a constant dependent on the type of resist used, λ is the wavelength of the exposure radiation (nm), and NA is the numerical aperture of the lens.
The resolution limit of currently available lithographic techniques is being reached due to a decrease in the depth of focus, difficulty in the design of lenses and complexities in the lens fabrication technology. A lower limit on the value of the constant k1 is approximately 0.25.
Currently available RET include optical proximity correction (OPC) of optical proximity errors (OPE), phase shifting masks (PSM), sub-resolution assist features (SRAF), and off-axis illumination (OAI). Each technique may be used alone, or in combination with other techniques.
When pattern dimensions approach the Rayleigh limit, the projected image is no longer a faithful reproduction of the mask pattern shaped. Optical proximity effects, such as corner rounding, line-end shortening, and line width errors are common problems in sub-resolution lithography. OPC techniques are used to pre-distort the mask pattern so that the shape of the projected image takes on a desired shape. Currently used OPC techniques include line-end treatments, for example serifs and hammerheads, that stretch out the ends of lines on the mask so that the final imaged lines have the desired length and sharpened corners. Line biasing adjusts the width of lines on the mask to compensate for variations of line width across pitch. Scattering bars are sub-resolution features that are placed alongside isolated or semi-isolated lines to help the lines behave more like dense lines, even through focus.
Phase shifting masks alter the phase of radiation passing through different portions of the mask, thus reducing diffraction effects by combining diffracted radiation and phase shifted diffracted radiation so that constructive and destructive interference takes places favorably, which increases contrast and improves resolution. Attenuated phase shift masks, chromeless phase lithography masks, and alternating phase shift masks are currently used to improve resolution of lithographic projection apparatus.
Sub-resolution assist features are added to masks in proximity to pattern features to improve resistance to process variations. The assist features image but do not print on the wafer. Assist features require a minimum amount of space to be effective, thus limiting their use in dense patterns.
Various illumination techniques, including off-axis illumination, may be used to improve resolution. Off-axis illumination improves resolution by illuminating the mask with radiation that is at an angle to the optical axis of the lens. The incidence of the radiation on the mask, which acts as a diffraction grating, improves the contrast of the image by transmitting more of the diffracted orders through the lens. Off-axis illumination techniques used with conventional masks produce resolution enhancement effects similar to resolution enhancement effects obtained with phase shifting masks.
Referring to FIGS. 2-5, currently available illumination intensity distributions or arrangements include small, or low, sigma (FIG. 2), annular (FIG. 3), quadrupole (FIG. 4), and quasar (FIG. 5), with the illuminated areas shown in cross section. The annular, quadrupole and quasar illumination techniques of FIGS. 3-5 are examples of off-axis illumination schemes. A lithographic apparatus capable of producing each off-axis illumination scheme shown in FIGS. 3-5 is disclosed in U.S. Pat. No. 6,452,662, incorporated herein by reference in its entirety.
Small sigma illumination is incident on the mask with approximately zero illumination angle (i.e. almost perpendicular to the mask) and produces good results with phase shifting masks to improve resolution and increase the depth of focus. Annular illumination is incident on the mask at angles that are circularly symmetrical and improves resolution and increases depth of focus while being less pattern dependent than other illumination schemes. Quadrupole and quasar illumination are incident on the mask with four main angles and provide improved resolution and increased depth of focus while being strongly pattern dependent.
Referring to FIGS. 6 and 7, two conventional illumination systems IL are
schematically illustrated. The systems illustrated in FIGS. 6 and 7 include light collecting/collimating optics 10; an axicon/zoom module 12; and light integrating and projecting optics 14. The illumination systems IL define an optical axis 16, a pupil plane 18, and a mask plane 20. The axicon/zoom module 12 comprises a pair of axicons 22, one concave and one convex, whose separation can be varied. The module 12 also comprises a zoom lens 24.
For the case of conical axicons, some examples of the illumination intensity distributions achievable at the pupil plane 18 are shown in FIG. 8. The spot size can be varied between states A and B by changing the zoom lens position. Similarly, the annularity can be changed between states A and C by varying the axicon opening (separation between the axicons).
To improve the illumination homogeneity, an optical integrator 26 is used. In FIG. 6 the optical integrator takes the form of a light pipe 26, such as a glass, calcium fluoride or quartz rod. A coupler 28 couples the illumination at the pupil plane 18 into the rod 26, and rod exit imaging optics 30 are also provided. In FIG. 7 a fly's eye element 32 acts as the integrator. The fly's eye element 32 is a composite lens comprising an array or honeycomb of small lenses. Further objective lenses 34, 36 complete the projection optics.
Resists are used to create positive and negative images of a desired pattern. A positive resist is one which becomes soluble after exposure to a specified dose of radiation. The pattern is formed in the resist by exposure and then developed, or removed, by a solvent, or an acid or base solution. After removal of the soluble resist, the exposed portion of the wafer is then etched and the etched pattern is filled with material, for example metal, to form the device. The remaining resist is then stripped to complete manufacture of the device.
Negative resists becomes insoluble after exposure to a dose of radiation. The solvent removes the unexposed (soluble) resist and leaves the exposed (insoluble) resist on the wafer in the shape of the desired pattern.
It is possible to print negative images by performing image reversal of positive resist. Such a process is disclosed, for example, in U.S. Pat. No. 4,104,070. The process generally includes coating a substrate or wafer with positive resist, pre-exposure soft baking the coated wafer, exposing the coated wafer to a patterned source of radiation, post-exposure baking the coated wafer at a temperature higher than the pre-exposure soft bake to cross link the exposed resist, flood exposing the coated wafer to make the previously unexposed resist dissolvable in the developer, and developing the resist.
Image reversal provides all of the advantages of positive resist, including improved resolution, superior edge quality, safer and more reliable processing with aqueous-based developers instead of flammable organic developers, and superior critical dimension (CD) control and uniformity. The image reversal process includes several disadvantages, however. The catalysts added to positive resists are not stable and must be prepared immediately before exposure. The catalysts added to the positive resist may also contaminate the resist and increase defect densities. The image reversal process also requires between two and three times the exposure dose of standard positive resist processing. The processes have not been extensively used or sufficiently developed for use in modern lithographic problems.
Negative imaging is used in the production of printed wiring boards. Negative imaging is also used in the production of III-V devices, such as those with GaAs substrates, for telecommunication device fabrication. A lift-off process is used in the fabrication of III-V devices due to the incompatibility of the substrate with etchable metals, such as Al, Au, Ni, Pt, Ta, and Ti. In the lift-off process a negative image is formed in a sacrificial resist layer deposited on a substrate, a metal film is deposited over the sacrificial resist layer and in the openings of the pattern on the substrate, and the portions of the metal film deposited on the sacrificial resist layer are removed (i.e. lifted-off) when the substrate is immersed in a solvent. The metal deposited through the openings of the pattern on the substrate is left behind in the desired pattern.
Negative imaging is also used in the production of lithographically defined, magnetic nanoparticles, or nanomagnets, for continuous magnetic data storage media having ultrahigh data density storage. Negative imaging is preferred to form the dimensions of holes patterned between neighboring magnetic pillars. Negative imaging yields three times the contrast of positive imaging for the generation of holes in resist, thus providing an enhanced process latitude for patterning gratings in which the lines are wider than the spaces. Currently available techniques permit creation of large area arrays of nanomagnets with dimensions in the 100 nm range, using either electroplating or lift-off processes.